1. Field of the Invention
The present invention relates to an electron microscope capable of analyzing the electronic state of a solid specimen by electron energy-loss spectroscopy (EELS).
2. Description of Related Art
When incident electrons collide against atoms constituting a solid specimen, some of the electrons interact with electrons within the specimen crystal or crystal lattice, lose part of their energy, and are scattered, that is, their velocity decreases. These scattered electrons are known as inelastically scattered electrons. A technique for analyzing the electronic state of a solid specimen by analyzing inelastically scattered electrons in terms of energy is known as electron energy-loss spectroscopy (EELS). On the other hand, when incident electrons collide against atoms constituting a solid specimen and scatter without losing energy, the electrons are termed elastically scattered electrons.
In the current electron microscopy, specimens are analyzed by the aforementioned EELS. FIG. 1A shows an EELS profile obtained by an EELS analysis. This profile has been obtained by image-processing a spectral image shown in FIG. 1B. This spectral image has been taken by a CCD camera disposed behind the projector lens. A method of gaining this spectral image shown in FIG. 1B is described below.
To obtain this spectral image, an electron microscope equipped with an energy filter (such as an Omega filter) behind a specimen is used. In this microscope having the energy filter, a sharply focused electron beam is directed at the specimen. As a result, electrons are ejected from the specimen. The ejected electrons are energy-dispersed in a given direction (i.e., direction substantially perpendicular to the direction in which electrons hit the specimen) by an energy filter. This energy-dispersed electron beam creates the spectrum shown in FIG. 1B between the energy filter and the projector lens. The spectrum is projected onto the CCD camera by the projector lens.
A method of gaining the spectral image shown in FIG. 1B has been described so far. The arrow E shown in the spectral image of FIG. 1B indicates the direction of energy dispersion. This direction E is coincident with the longitudinal direction of the spectrum.
The spectral image obtained in this way is converted into the EELS profile shown in FIG. 1A by an image processor. In particular, the processor accumulates the intensity of the obtained spectral image in a direction perpendicular to the direction of energy dispersion E. The intensity variation is plotted against the direction of energy dispersion, thus giving rise to an EELS profile (see Japanese Patent Laid-Open No. 2001-76664). A sharp peak P0 appearing on the EELS profile shown in FIG. 1A is a zero-loss peak at which energy loss is zero. This zero-loss peak P0 is produced by the aforementioned elastically scattered electrons.
Where some surface of a specimen is analyzed by EELS, the electron beam hitting the specimen is deflected in the x- and y-directions by deflectors. The specimen surface is scanned in two dimensions by the sharply focused beam. The aforementioned spectral image is obtained at each analysis point on the specimen irradiated with the beam. Hence, an EELS profile at each analysis point on the sample is obtained. The electronic state of some surface of the specimen can be analyzed from these EELS profiles.
However, this surface analysis using EELS produces the following problems: (1) When the electron beam scans over the specimen surface, the spectral position on the CCD camera is moved. The amount of movement of the spectrum is in proportion to the scanning width of the beam on the specimen surface. Where the electron beam is deflected to a great extent and a wide area of the specimen surface is scanned by the beam, the spectrum does not lie within the light-sensitive surface of the CCD camera. (2) Since the spectral position moves across the CCD camera as mentioned previously, the spectral positions within plural spectral images taken by the CCD camera are not fixed. Consequently, the EELS profile cannot be calibrated in the direction of energy dispersion unless the image position (position p in FIG. 1B) corresponding to the zero-loss peak P0 is detected by an image recognition technique for every spectral image obtained at each analysis point. Since the image recognition processing is time-consuming, it has been heretofore difficult to display EELS profiles at high speed. (3) Where the spectral position moves across the CCD camera and the spectral part forming the zero-loss peak P0 comes out of the light-sensitive surface of the camera as described above, it is impossible to determine the energy position of the EELS profile by the image recognition technique.